1. Field of the Invention
The present invention relates to a fixing means to fix an internal gear wheel to a casing in a planetary gear type reduction gear device.
2. Discussion of Background
There has been known the planet gear type reduction gear device of this kind disclosed, for instance, in Japanese Unexamined Patent Publication No. 82046/1986 or Japanese Unexamined Utility Model Publication No. 49070/1986. FIG. 3 is a cross-sectional view showing such a device. In FIG. 3, a reference numeral 1 designates a planet gear supported by a support pin 3 through a sleeve bearing 2, which effects a planetary movement described hereinafter. A numeral 4 designates a sun gear disposed at the central portion of the reduction gear device, a numeral 5 designates an internal gear wheel disposed outside the planet gear 1. A projection 6 is formed integrally with at an outer circumferential surface in the radial direction of the internal gear wheel 5. The internal gear wheel 5 is fixed to the inside of the casing 7 by engaging the projection 6 with a recess 8 formed in the internal circumferential surface of the casing 7.
The operation of the planet gear type reduction gear device having the above-mentioned construction will be described. When a rotating force is applied to the sun gear 4, the planet gear revolves around the sun gear 4 and the inner circumferential surface of the internal gear wheel 5 because the planet gear 1 is meshed with the sun gear 4 and the internal gear wheel 5, and also the planet gear 1 rotates on the support pin 3. In this case, there is no relative movement of the internal gear wheel 5 to the casing 7 because the internal gear wheel 5 is fixed to the casing due to the engagement of the projection 6 with the recess 8, and only the planet gear 1 meshed with the internal gear wheel 5 revolves while it rotates. Thus, the rotating force of the sun gear 4 is transmitted to the support pin 3 so that the support pin 3 rotates at a reduction ratio of ##EQU1## (where Z.sub.1 is the number of teeth of the sun gear 4 and Z.sub.3 is the number of teeth of the internal gear wheel 5). As described above, a function of speed-reducing can be obtained.
The conventional planet gear type reduction gear device had such a problem that there was a restriction in dimension in the radial direction of the reduction gear device because of the construction of the fixing means which secures the internal gear wheel 5 to the casing 7 by the engagement of the projection 6 formed at the outer surface in the radial direction of the internal gear wheel 5 with the recess 8 formed in the inner circumferential surface of the casing 7. The outer diameter of the casing 7 is determined in designing a planet gear type reduction gear device. When the number of teeth of the internal gear wheel 5 of the device is to be increased, it is necessary to reduce the wall thickness of the bottom portions of the teeth of the internal gear wheel 5 or to reduce the wall thickness of the casing 7. However, when the wall thickness of the casing 7 is to be reduced, the thickness of the portion where the recess 8 is formed in the casing 7 becomes thinner, whereby a sufficient strength can not be obtained. Further, in a case that the internal gear wheel 5 is made of a resinous material, the wall thickness of the internal gear wheel 5 has to be maintained to provide a strength. On the other hand, when a sufficient strength is to be maintained, the outer diameter of the casing 7 is inevitably large. When there is a restriction in the dimension of the outer diameter in a planet gear type reduction gear device, it is difficult to maintain a predetermined strength.